Electrophilic additions to indene and indenone: factors effecting syn addition

Abstract

The electrophiles bromine, bromine chloride, chlorine, acetyl hypochlorite, and acetyl hypobromite all yield substantial cis addition product by reaction with indene (1) in aprotic solvents, showing that the direct syn collapse of ion pairs obtained from bromine and similar electrophiles is readily possible. The cis to trans ratios with the halogens (but not with the acyl hypohalites) vary with solvent polarities. In the low-polarity solvents (hydrocarbons and carbon tetrachloride) much more syn addition occurs at high halogen concentration. Syn addition of the above electrophiles to 1 as well as methyl hypobromite and methyl hypochlorite was also observed in methanol and acetic iscid, with the exception of bromination in methanol where only the trans-dibromide was obtained. Bromine and bromine chloride addition to indenone (20) yielded only the trans-dihalides, suggesting that a neighboring keto group stabilizes the bridged bromonium ion. Chlorination of 20 was not stereospecific but yielded more trans-dichloride than did chlorination of 1.